Although there may be some overlap and Simple IoT may eventually support a number of off the shelf consumer IoT devices, the genesis and intent of the project is for developing IoT products and the infrastructure required to support them.
Particle.io provides excellent infrastructure to support their devices and solve many of the hard problems such as remote FW update, getting data securely from device to cloud, efficient data bandwidth usage, etc. But they don't provide a way to provide a user facing portal for a product that customers can use to see data and interact with the device.
SIOT is designed to be simple to develop and deploy without a lot of moving parts. We've reduced an IoT system to a few basic concepts that are exactly the same in the cloud and on edge devices. This symmetry is powerful and allows us to easily implement and move functionality wherever it is needed. If you need Google Scale, SIOT may not be the right choice; however, for smaller systems where you want a system that is easier to develop, deploy, and maintain, consider SIOT.
This is a good question and I'm not sure yet. NATS has some very interesting features like Jetstream which can queue data and and store data in a key-value store and data can be syncronized between instances. NATS also has a concept of leaf-nodes, which conceptually makes sense for edge/gateway connections. Jetstream is optimized for data flowing in one direction (ex: orders through fullfillment). SIOT is optimized for data flowing in any direction and data is merged using data structures with CRDT (conflic-free replicated data types) properties. SIOT also stores data in a DAG (directed acyclic graph) which allows a node to be a child of multiple nodes, which is difficult to do in a hiearchical namespace. Additionally, each node is defined by an array of points and modifications to the system are communicated by transferring points. SIOT is a batteries included complete solution for IoT solutions, including a web framework, clients for various types of IO (ex: Modbus) and cloud services (ex: Twilio). We will continue to explore using more of NATS core functionality as we move forward.